I'm almost 43, and my youngest is 9. Sometimes I feel like an older mom, but other times I don't. It depends on where I am. The school district where my boys go to school is an affluent district, and most of the parents started their careers before having children, so whenever I'm around them I feel like I'm right on track.
The school district where I teach has many low-income families, and many of them had children younger. Many of my high school students have parents who are much, much younger than me. I feel a bit old when I'm around them, but it doesn't bother me anymore. I guess I've gotten used to it.
My oldest son is 15; I had him when I was 28, and I definitely had more energy when he was younger than I do with my youngest son. On a more positive note, though, we are much more financially stable, now, and are more confident in our parenting than we would have been if we had had our boys when we were younger. My husband is 50, and he complains about being an older dad sometimes, but when we reflect back on our lives there really isn't anything that we would change.
One thing I will add, I felt very old when I turned 40. It was more about dealing with my mortality than anything. Suddenly I realized I wasn't going to live forever and it totally freaked me out. That lasted for several months, and then I came to grips with my age, and since then I have loved my 40s.
Good luck working through your feelings! :)